Provide xmacro for initializing PyObject strings

Change-Id: Icc3bea8345c0051d3e34e2ec1716b18a5208e32e
Reviewed-on: http://review.couchbase.org/26607
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>